1. Field of the Invention
The present invention concerns a method of measuring an electric current in a plurality of conductors and a device for implementing such a method.
The invention finds a particularly advantageous application in the field of rotary electrical machines used in the automotive industry.
2. Description of the Related Art
Motor vehicles with a thermal engine can be equipped with reversible electrical machines, also referred to as alternator/starters, functioning both in alternator mode and in motor mode on starting up or as an aid to boosting as from 500 revolutions/min for the thermal engine.
The reversible electrical machine also comprises a power unit and a control unit, the power unit serving as a current inverter in starter mode and as current rectifier in alternating mode and is controlled by the control unit.
In this type of machine, it is necessary to be able at all times to control the torque supplied or taken off by the alternator/starter. However, this torque depends directly on the current of the stator of the machine and more precisely on the currents at the various phases of the stator when the latter is functioning in multiphase current, for example three-phase. In order to monitor and subsequently regulate the various stator currents, there therefore exists a current regulation unit that is generally numeric.
In the machine, these currents pass through conductors, generally with a high cross section, placed between the stator and a rectifier, or inverter, on the power unit.
These conductors are for example parallel rectilinear conductors referred to as bus-bars.
The advantage, for a control of the alternator/starter most appropriate to the functioning of the vehicle engine, of knowing precisely the currents passing over the n bus-bars, n being equal to 3 in three-phase machines, will therefore be understood.
In order to determine the stator currents, recourse is had to various known types of device.
FIG. 1 is a side view of a known device using a magnetic circuit CM made from ferrite surrounding each conductor CO and closing over a Hall effect sensor CA that measures the magnetic field used by the current I passing through the conductor CO, the sensor being opposite the magnetic circuit. Such a device is fixed to a dissipater of the alternator/starter power unit.
However, these devices are expensive, bulky and complex to use since they require firstly a connecting link between the sensor and the control unit normally referred to as a control card and secondly passing each conductor of bus-bar through a ferrite magnetic circuit.
Devices based on electrical shunts are also known, which are however not well suited if it is wished to measure very intense currents (800 A for example) with few losses by Joule effect. In addition, there exist problems of connection to the conductors. Finally, in the case of low shunt values there is a great deal of impreciseness with regard to the measurements of the low currents.